1. Field of the Invention
This invention relates to a display apparatus, a driving method for a display apparatus and an electronic apparatus, and more particularly to a display apparatus of the flat type or flat panel type wherein a plurality of pixels each including an electric optical device are disposed in rows and columns, that is, in a matrix, and a driving system for a display apparatus of the type described and an electronic apparatus which includes a display apparatus of the type.
2. Description of the Related Art
In recent years, in the field of display apparatus for displaying an image, display apparatus of the flat type wherein a plurality of pixels or pixel circuits each including a light emitting device are arranged in rows and columns, that is, in a matrix, have been popularized rapidly. As one of such display apparatus of the flat type, an organic EL (Electro Luminescence) display apparatus has been developed and commercialized. The organic EL display apparatus uses an organic EL device as a light emitting device for pixels. The organic EL device is an electro-optical device of the current-driven type whose light emission luminance varies in response to the value of current flowing through the device, and particularly utilizes a phenomenon that an organic thin film emits light if an electric field is applied thereto.
The organic EL display apparatus has the following characteristics. In particular, since the organic EL device can be driven with an applied voltage equal to or lower than 10 V, it is low in power consumption. Further, the organic EL device is a self-luminous device. Therefore, in comparison with a liquid crystal display apparatus wherein the intensity of light from a light source called backlight is controlled by a liquid crystal cell included in each pixel to display an image, the organic EL display apparatus is superior in visual observability of a display image. Besides, since the organic EL display apparatus does not demand an illuminating member such as a backlight which is essentially required by the liquid crystal display apparatus, it is easy to reduce the size and thickness. Further, since the response speed of the organic EL device is as high as approximately several microseconds, no after image appears upon display of moving pictures on the organic EL display apparatus.
The organic EL display apparatus can adopt a simple (passive) matrix system and an active matrix system as a driving system similarly to the liquid crystal display apparatus. However, although the display apparatus of the simple matrix type is simple in structure, it has such a problem that it is difficult to implement the same as a display apparatus of a large size and a high definition because the light emission period of the electro-optical device decreases as the number of scanning lines, that is, the number of pixels, increases.
Therefore, in recent years, development of display apparatus of the active matrix system has been and is being carried out energetically wherein current flowing through an electro-optical device is controlled by an active device provided in a pixel circuit which includes the electro-optical device, for example, an insulated gate type field effect transistor, usually a TFT (Thin Film Transistor). The display apparatus of the active matrix system can be implemented readily as a display apparatus of a large size and a high definition since light emission of the electro-optical device continues over a period of one frame.
Incidentally, it is generally known that the I-V characteristic, that is, the current-voltage characteristic, of the organic EL device deteriorates as time passes. Such deterioration is called age deterioration or age-related deterioration. In a pixel circuit which uses an N-channel type TFT as a transistor for current driving an organic EL device (such a transistor is hereinafter referred to as “driving transistor”), the organic EL device is connected to the source side of the driving transistor. Therefore, if the I-V characteristic of the organic EL device exhibits age deterioration, then the gate-source voltage Vgs of the driving transistor varies, resulting in variation also of the light emission luminance of the organic EL device.
This is described more particularly. The source potential of the driving transistor depends upon the working point of the driving transistor and the organic EL device. If the I-V characteristic of the organic EL device deteriorates, then the operating point of the driving transistor and the organic EL device varies, and consequently, even if the same voltage is applied to the gate of the driving transistor, the source potential of the driving transistor varies. Since this varies the source-gate voltage Vgs of the driving transistor, the value of current flowing through the driving transistor varies. As a result, the value of current flowing through the organic EL device varies, and this varies the light emission luminance of the organic EL device.
On the other hand, a pixel circuit which uses a polycrystalline silicon TFT suffers, in addition to age deterioration of the I-V characteristic of the organic EL device, from age deterioration of the threshold voltage Vth of the driving transistor or the mobility μ of a semiconductor thin film which forms a channel of the driving transistor (such mobility is hereinafter referred to as driving transistor mobility). Further, in the pixel circuit, the threshold voltage Vth or the mobility μ differs among different pixels or individual transistor characteristics have a dispersion thereamong due to a dispersion in the fabrication process.
If the threshold voltage Vth or the mobility μ of the driving transistor differs among different pixels, then this gives rise to a dispersion in value of current flowing through the driving transistor among different pixels. Therefore, even if the same voltage is applied to the gate of the driving transistor among the pixels, a dispersion appears in the light emission luminance of the organic EL device among the pixels, and this degrades the uniformity of the screen image.
Therefore, in order to keep the light emission luminance of the organic EL device fixed without being influenced even if the I-V characteristic of the organic EL device exhibits age deterioration or the threshold voltage Vth or the mobility μ of the driving transistor exhibits age variation, correction functions are provided for each pixel circuit. In particular, a compensation function for compensating for the characteristic variation of the organic EL device, a function for carrying out correction against the variation of the threshold voltage Vth of the driving transistor (such correction is hereinafter referred to as “threshold value correction”) and a function for carrying out correction against the variation of the mobility μ of the driving transistor (such correction is hereinafter referred to as “mobility correction”) are provided for each pixel circuit. A pixel circuit of the configuration described is disclosed, for example, in Japanese Patent Laid-Open No. 2006-133542 (hereinafter referred to as Patent Document 1).
Where each pixel circuit includes the compensation function for the characteristic variation of the organic EL device and the correction functions against the variation of the threshold voltage Vth and the mobility μ of the driving transistor in this manner, even if the I-V characteristic of the organic EL device exhibits age deterioration or the threshold voltage Vth or the mobility μ of the driving transistor exhibits age variation, the light emission luminance of the organic EL device can be kept fixed without being influenced by such age deterioration or age variation.